1. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to switching between two antennas.
2. Description of the Related Art
Radiotelephones can be of the portable type that use a small battery to power the radiotelephone. These units typically operate at a low power output due to the limited power output of the battery. The limited battery life is a trade-off to obtain a smaller, more portable radiotelephone.
Mobile radiotelephones are typically powered by a vehicle's electrical system. This enables the mobile radiotelephone to operate at a higher power output since battery output is not a concern. The drawback of the mobile is the lack of portability. The mobile is typically required to remain connected to the vehicle.
Vehicle adapter kits have been developed giving portable radiotelephones the benefits of mobile radiotelephones. The portable is connected to the vehicle adapter kit when the user is in the vehicle. This connection allows the portable to be powered by the vehicle electrical system.
Additionally, when the portable radiotelephone is connected to the vehicle adapter, the mobile's antenna is used due to its higher gain and better placement above the vehicle. In order to accomplish this, the coupling of the portable to the vehicle adapter must also switch from communicating with the portable antenna to using the mobile antenna.
This switching from one antenna to the other is accomplished by a solid state relay in the portable radiotelephone. When the portable is connected to the vehicle adapter, the relay is energized and switched such that the portable's antenna is switched out and the vehicle's antenna is switched in. This allows the portable's transceiver to communicate over the vehicle antenna.
This relay, however, increases the noise figure of the radiotelephone's front end. This results in increased distortion to the signals being communicated with a resulting loss of data or voice. There is a resulting need for a way to switch from a portable's internal antenna to the vehicle antenna without causing distortion of transmitted and received signals.